This invention relates to an apparatus for generating a beam of charged particles, particularly to an ion gun for use in an ion beam processing apparatus and to an ion beam processing apparatus incorporating same.
Ion beams have been used for many years in the production of components in the micro-electronics industry and magnetic thin film devices in the storage media industry. Typically, an ion beam, such as an argon ion beam, is required to have a large area, a high current and an energy of between 100 eV and 2 keV. The beam can be used in a number of ways to modify the surface of a substrate, for example by sputter deposition, sputter etching, milling, or implantation.
In a typical ion beam source (or ion gun) a plasma is produced by admitting a gas or vapour to a low pressure discharge chamber containing a heated cathode and an anode which serves to remove electrons from the plasma and to give a surplus of positively charged ions which pass through a screen grid or grids into a target chamber which is pumped to a lower pressure than the discharge chamber. Ions are formed in the discharge chamber by electron impact ionisation and move within the body of the ion gun by random thermal motion. The plasma will thus exhibit positive plasma potential which is higher than the potential of any surface with which it comes into contact. Various arrangements of grids can be used, the potentials of which are individually controlled. In a multigrid system the first grid encountered by the ions is usually positively biased whilst the second grid is negatively biased. A further grid may be used to decelerate the ions emerging from the ion source so as to provide a collimated beam of ions having more or less uniform energy. For ion sputtering a target is placed in the target chamber where this can be struck by the beam of ions, usually at an oblique angle, and the substrate on to which material is to be sputtered is placed in a position where sputtered material can impinge on it. When sputter etching, milling or implantation is to be practised the substrate is placed in the path of the ion beam.
Hence, in a typical ion gun an ion arriving at a multiaperture extraction grid assembly first meets a positively biased grid. Associated with the grid is a plasma sheath. Across this sheath is dropped the potential difference between the plasma and the grid. This accelerating potential will attract ions in the sheath region to the first grid. Any ion moving through an aperture in this first grid, and entering the space between the first, positively biased grid, and the second, negatively biased, grid is strongly accelerated in an intense electrical field. As the ion passes through the aperture in the second grid and is in flight to the earthed target it is moving through a decelerating field. The ion then arrives at an earthed target with an energy equal to the potential of the first, positive, grid plus the sheath potential.
Hence, a conventional ion gun comprises a source of charged particles which are accelerated through an externally applied electric field created between a pair of grids. Conventionally, for low energy ion beam production, three grids are used, the first being held at a positive potential, the second being held at a negative potential adjusted to give the best divergence, and the third, if present, at earth potential, i.e. the potential of the chamber in which the beam is produced. Beams of this nature are well described in the open literature going back over 25 years.
In some applications it is desirable to obtain an ion beam of maximum current. However, in other applications it is the divergence of the ions comprising the beam, or rather the relative lack of it, that is critical to achieving a suitable process performance.
U.S. Pat. No. 4,447,773 discloses an ion beam accelerator system for extracting and accelerating ions from a source. The system includes a pair of spaced, parallel extraction grids, 60 mil (1.524 mm) apart, having aligned pairs of holes for extracting ion beamlets. The pairs of holes are positioned so that the beamlets converge and the merged beamlets are accelerated by an accelerator electrode which is 0.6 inch (15.24 mm) downstream of the extraction grid pair. The extraction grids are formed with numerous small holes through which beamlets of ions can pass and are maintained a potential difference of a few hundred volts. The accelerator electrode has a single hole, which is slightly greater in height than the height of the matrix of holes in the extraction grids, and is maintained at a much lower potential for accelerating the converged ion beam emerging from the extraction grid pair.
An extensive introduction to and prior art review of ion beam technology is provided in EP-B-0462165. EP-B-0462165 itself describes an ion gun in which the plasma from which the ions are accelerated by the accelerator grid is at a low potential of not more than about 500 V and is of uniform density so as to permit high current densities of the order 2 to 5 mA/cm2 in the ion beam to be achieved at low potential (i.e. less than about 500 V), and with minimum risk of damage to the accelerator grid, in operation. This system provides an ion gun in which the plasma can be efficiently generated using a commercially acceptable high radio frequency, such as 13.56 MHz or a multiple thereof, and in which the resulting plasma has the desirable properties of high density, good uniformity and a relatively low plasma potential.
However, a perennial problem with the ion beam sources described in the prior art is the high magnitude of divergence to which the beam is susceptible. Whilst the system of EP-B-0462165 solves many of the problems associated with other prior art ion beam sources, it would still be highly desirable to improve this system to provide a reduced degree of ion beam divergence. There is a growing demand for ultra low divergence at low to medium beam energy. In this role the beam is typically described as an ion mill selectively etching deep trenches of perhaps 1xc3x9710 xcexcm scale length. To do so requires a beam with a divergence of no more than approximately 1xc2x0 at an energy dictated by the constraints of maximum rate at a processing energy of perhaps as low as 500 eV. Conventional ion guns operating at this energy cannot meet the divergence requirement at a high enough current to meet the process rate.
The present invention accordingly seeks to provide an ion gun which is capable of operation in a manner such that the above aims are substantially achieved.
According to the present invention there is provided apparatus for the production of low energy charged particle beams comprising: a plasma chamber; means for generating in the plasma chamber a plasma comprising particles of a first polarity and oppositely charged particles of a second polarity; means for restraining particles of the first polarity in the plasma chamber; a first multi-aperture electrode grid contacting the plasma, wherein the first electrode grid is arranged for connection to a first potential source so as to impart to the first electrode grid a first potential of the second polarity; a second multi-aperture electrode grid arranged for connection to a second potential source so as to impart to the second electrode grid a second potential, the second potential being less than in the sense of being either less positive than or less negative than the first potential so as to produce between the first and second electrode grids a first acceleration field for accelerating charged particles of the second polarity towards and through the second grid; and a third multi-aperture electrode grid arranged for connection to a third potential source so as to impart to the third electrode grid a third potential of the first polarity and to produce between the second and third electrode grids a second acceleration field for accelerating charged particles of the second polarity towards and through the third electrode grid, the grid spacing between the first and second grids being greater at the periphery of the grids than at the centre thereof, the apertures of the first, second and third grids being aligned so that particles emerging from an aperture of the first grid are accelerated through a corresponding aperture of the second grid and then through a corresponding aperture of the third grid in the form of a beamlet, a plurality of beamlets from the third grid forming a beam downstream of the third grid.
The apparatus of the invention may be used to generate an electron beam. in which case the charged particles of the first polarity are ions and the charged particles of the second polarity are electrons, or to generate an ion beam, in which case the charged particles of the first polarity are electrons and the charged particles of the second polarity are ions.
Accordingly, the invention provides a low energy ion gun for use in ion beam processing comprising: a plasma chamber comprising an open ended, conductive, non-magnetic body, a first end of which is closed by a flat or minimally dished dielectric member and with electrodes at a second end thereof opposite the first end; primary magnet means arranged around the body for trapping electrons adjacent the wall of the plasma chamber in use of the ion gun; and an r.f. induction device including a substantially flat coil which lies adjacent to the dielectric member for inductively generating a plasma in the plasma chamber, characterised in that the electrodes include a first multi-aperture grid arranged for connection to a first positive potential source and positioned to contact the plasma in the plasma chamber; a second multi-aperture grid arranged for connection to a second potential source of lower potential than the first source so as to produce a first acceleration field for accelerating ions towards and through the second grid; and a third multi-aperture grid arranged for connection to a third potential source of lower potential than the second potential source so as to produce a second acceleration field for accelerating ions towards and through the third grid, the grid spacing between the first and second grids being greater at the periphery of the grids than at the centre thereof, the apertures of the first, second and third grids being aligned so that particles emerging from an aperture of the first grid are accelerated through a corresponding aperture of the second grid and then through a corresponding aperture of the third grid in the form of a beamlet, a plurality of beamlets from the third grid forming a beam downstream of the third grid.
The provision of a grid spacing between the first and second grids which is greater at the periphery of the grids than at the centre thereof is an important feature of these embodiments of the invention. Preferably this variation in grid spacing is achieved by contouring one or both of the neighbouring surfaces of the first and second grids. Thus, in one preferred embodiment, the second grid has a generally flat surface towards its periphery but in its central region bulges outwardly towards the first grid. The provision of this variation in grid spacing over the grids recognises that the plasma density of the beam approaching the first grid tends to diminish towards the periphery of the beam. The acceleration field to which individual beamlets are subject on passing through the first grid depends to some extent upon the grid spacing, which may therefore be selected to optimise the divergence of individual beamlets, whether from the periphery or the central region of the first grid.
In one embodiment of the invention, the third potential source may be arranged to impart a negative potential to the third grid. Alternatively, the third potential source may be arranged to earth the third grid. In this case, a fourth grid may be provided and arranged for connection to earth.
The invention further provides a low energy ion gun for use in ion beam processing comprising: a plasma chamber comprising an open ended, conductive, non-magnetic body, a first end of which is closed by a flat or minimally dished dielectric member and with electrodes at a second end thereof opposite the first end; primary magnet means arranged around the body for trapping electrons adjacent the wall of the plasma chamber in use of the ion gun; and an r.f. induction device including a substantially flat coil which lies adjacent to the dielectric member for inductively generating a plasma in the plasma chamber, characterised in that the electrodes include a first multi-aperture grid arranged for a connection to a first positive potential source and positioned to contact the plasma in the plasma chamber; a second multi-aperture grid arranged for connection to a second potential source of lower potential than the first source so as to produce a first acceleration field for accelerating ions towards and through the second grid; a third multi-aperture grid arranged for connection to a negative potential source so as to produce a second acceleration field for accelerating ions towards and through the third grid; and a fourth multi-aperture grid arranged for connection to earth, the apertures of the first, second, third and fourth grids being aligned so that ions emerging from an aperture of the first grid are accelerated through a corresponding aperture of the second grid and then through a corresponding aperture of the third grid before passing through a corresponding aperture of the fourth grid in the form of a beamlet, a plurality of beamlets from the fourth grid forming a beam downstream of the fourth grid.
Also provided in accordance with the invention is a low energy electron gun for use in electron beam processing comprising: a plasma chamber comprising an open ended, conductive, non-magnetic body, a first end of which is closed by a flat or minimally dished dielectric member and with electrodes at a second end thereof opposite the first end; primary magnet means arranged around the body for trapping ions adjacent the wall of the plasma chamber in use of the electron ion gun; and an r.f. induction device including a substantially flat coil which lies adjacent to the dielectric member for inductively generating a plasma in the plasma chamber, characterised in that the electrodes include a first multi-aperture grid arranged for connection to a first negative potential source and positioned to contact the plasma in the plasma chamber; a second multi-aperture grid arranged for connection to a second negative potential source of less negative potential than the first source so as to produce a first acceleration field for accelerating electrons towards and through the second grid; and a third multiaperture grid arranged for connection to a third potential source of higher potential than the second potential source so as to produce a second acceleration field for accelerating electrons towards and through the third grid, the apertures of the first, second and third grids being aligned so that electrons emerging from an aperture of the first grid are accelerated through a corresponding aperture of the second grid and then through a corresponding aperture of the third grid in the form of a beamlets, a plurality of beamlets from the third grid forming a beam downstream of the third grid.
The invention also provides a method for generating a low energy ion beam, which method comprises;
(a) providing an ion gun according to the foregoing description;
(b) supplying to the plasma chamber a plasma forming gas;
(c) exciting the R.f. induction device to generate a plasma within the plasma chamber;
(d) supplying the plasma to an inlet end of the first grid so that the plasma passes through the first grid towards an outlet end thereof;
(e) accelerating the plasma between the outlet end of the first grid and an inlet end of the second grid so that the plasma passes through the second grid towards an outlet end thereof;
(f) further accelerating the plasma between the outlet end of the second positive grid and an inlet end of the third grid so that the plasma passes through the third grid towards an outlet end thereof; and
(g) recovering an ion beam from the outlet end of the third grid.
The invention further provides a low energy ion beam processing apparatus comprising
(1) a vacuum chamber;
(2) an ion gun arranged to project an ion beam into the vacuum chamber;
(3) an ion beam neutraliser for projecting electrons into the ion beam; and
(4) a support for a target or a substrate in the path of the ion beam; the ion gun comprising:
a plasma chamber comprising an open ended, conductive, non-magnetic body, a first end of which is closed by a flat or minimally dished dielectric member and with electrodes at a second end thereof opposite the first end;
primary magnet means arranged around the body for trapping electrons adjacent the wall of the plasma chamber in use of the ion gun; and
an r.f. induction device including a substantially flat coil which lies adjacent to the dielectric member for inductively generating a plasma in the plasma chamber, characterised in that the electrodes include a first multi-aperture grid arranged for connection to a first positive potential source and positioned to contact the plasma in the plasma chamber;
a second multi-aperture grid arranged for connection to a second potential source of lower potential than the first source so as to produce a first acceleration field for accelerating ions towards and through the second grid; and
a third multi-aperture grid arranged for connection to earth or to a negative potential source so as to produce a second acceleration field for accelerating ions towards and through the third grid, the apertures of the first, second and third grids being aligned so that particles emerging from an aperture of the first grid are accelerated through a corresponding aperture of the second grid and then through a corresponding aperture of the third grid in the form of a beamlet, a plurality of beamlets from the third grid forming a beam downstream of the third grid.
By xe2x80x9clow energyxe2x80x9d is meant up to about 10 kV, for example 5 kV or less. Usually, the ion beam generated by the apparatus of the invention will have an energy of 1 kV or less.
The ion gun of the invention is capable of generating an ion beam of significantly lower divergence than has conventionally been achievable. A 500 eV ion beam generated by a gun according to the invention may have a divergence of as little as 1xc2x0. This compares directly with values of between 3xc2x0 and 5xc2x0 for prior art ion beams utilising conventional three grid electrode grid structures. It has surprisingly been discovered that an underlying design rule for ultra-low divergence ion beams has not been recognised in the prior art. The basis of the prior art, as exemplified by EP-B-0462165, lies in the electrostatic lens principle underpinning the simple two/three grid conventional accelerator structures and its balance with the natural space charge repulsive force in the beam. This repulsive force leads to an irreducible divergence limit for such structures. The ion beams of the prior art are vigorously compressed by a strong accelerating force provided by a first, positively charged grid and a second, negatively charged grid. The potential difference between the first and second grids may be of the order of 1000 V. As the beam passes through the second grid, the space charge force reaches a maximum and acts upon the beam to cause it to diverge as it propagates beyond the second grid. The space charge force increases with increasing beam current, with reducing beam radius and with reducing beam energy. Empirically it has been found that the lower limit of divergence for a 500 eV beam for a three grid accelerator with a beam current viable for industrial processing lies between 3xc2x0 and 5xc2x0. In contrast, the ion gun of the present invention is capable of yielding a divergence value of 1xc2x0.
One preferred way in which the angular divergence of the beam may be minimised in the present invention is by adopting a more gentle acceleration field between the first and second grids than between the second and third grids. This allows the beamlets to propagate at a larger net area, hence reducing the space charge repulsion inside the accelerator grid structure itself.
The ion gun of the invention may of course be provided with more than three grids. For example, three, four or more positive biassed grids, each of successively lower positive bias than its upstream neighbour, could be used. Alternatively, or in addition, a plurality of negatively biassed or earthed grids could be incorporated towards the downstream end of the grid structure. However, for most applications, it is envisaged that a three or four grid structure will be preferred. In the three grid structure, the first and second grids may be positively biassed, the first grid being in contact with the plasma in the plasma chamber, the second grid being of lower positive bias than the first grid, and the third grid may be negatively biassed or earthed. In the four grid structure the first and second grids may be positively biassed, as described above, while the third grid may be negatively biassed and the fourth grid is earthed or negatively biassed. Thus, in one preferred embodiment of the invention, the third grid of the electrodes is arranged for connection to a negative potential source and the electrodes include a fourth grid arranged for connection to earth. The fourth grid may be used to provide an extra degree of control over the rates of acceleration and divergence of the ion beam.
Preferably, the grid arrangement is rigid since the mechanical separation of the grids plays a large part in determining the divergence of the beam. For example, a variation of 10% or more in the distance between two grids can have a significant impact on the net divergence of a large area beam. Furtherrnore, the relationship between beam divergence and the magnitude of the gap between the grids is substantially non-linear.
However, beam divergence is also a function of the local ion current density in the beam. As the cross-sectional area of the beam increases, the current density in the beam cross-section may vary by up to about 10%. The current density is lower towards the periphery of the beam.
In one embodiment of the invention, the grids are arranged in parallel alignment with each other. The gap between neighbouring grids is preferably between about 0.5 mm and about 3.0 mm, typically about 1.00 mm.
In a preferred embodiment, however, one grid of a neighbouring pair of grids is contoured so that the gap between the two grids of the pair is larger towards the periphery of the grids than towards the centre of the grids. For example, the gap at the centre of the pair may be about 1.00 mm while the gap at the periphery is about 1.3 mm. Preferably, the second grid is contoured.
The variation in current density across the beam is usually constant and repeatable and this may be exploited to obtain best average divergence across a large beam. Numerical simulation may be used to confirm this variation with respect to the magnitude of the in gap between neighbouring grids and the current density in the beam.
In one preferred embodiment of the invention, four grids are provided, the gap between each grid, at the centre thereof, being about 1.00 mm. Preferably, one or more of the grids is contoured as described above. Even more preferably, only the second grid is contoured.
The ion gun of the invention is of particular value for generating low energy beams of heavy ions, such as argon, which are commonly used in ion milling applications. Since the space charge force increases in inverse proportion to the ion velocity, the effect on the divergence of an argon ion beam at 500 eV is over 50 times larger than for a hydrogen ion beam at 50 KeV for a comparable beam current. Other heavy ions commonly used in ion milling applications include ions derived from krypton, xenon, H2, O2, Cl2, N2, CO2, SF6, C2F6 or a C2F6/CHF3 mixture.
In the ion gun of the present invention inductive r.f. coupling is used to generate a plasma in the plasma chamber. The resulting plasma typically exhibits a plasma potential that is no more than a few tens of volts above the potential of the plasma chamber or of the highest potential of the internal surface thereof. This is in contrast to many of the prior art designs of ion gun which utilise capacitative r.f. coupling to generate the plasma and which form a plasma with a plasma potential of some hundreds of volts.
The wall means may be constructed from an electrically conductive material. However, if it is desired, for example, to avoid any possibility of contamination of the ion beam by metallic ion contaminants, then the wall means may be constructed from a dielectric material.
The primary magnet means may comprise an array of magnets arranged to produce lines of magnetic flux within the plasma chamber which extend in a curve from the wall of the plasma chamber and return thereto so as to form an arch over a respective one of a plurality of wall regions of said plasma chamber, for example, wall regions which extend substantially longitudinally of the wall of the plasma chamber. Rare earth magnets are preferably used. Specific arrangements of the primary magnet means are described in EP-B-0462165 and are well understood by those skilled in the art.
It is preferred to use as near flat dielectric member as possible. Hence minimal dishing of the dielectric member is preferred. However, it may not be practical to avoid all dishing of the dielectric member as it must be ensured that the integrity of the vacuum equipment be preserved and that all risk of fracture of the dielectric member due to pressure differences exerted across it during operation is substantially obviated.
The r.f. emitter means associated with the dielectric member comprises a substantially flat spirally wound coil which preferably lies adjacent to, or is embedded within, the dielectric member. Hence the coil is preferably flat or as near flat as practicable. Such a coil may take the form of a tube of conductive material e.g. copper, through which a coolant, such as water, can be passed. This type of coil, and the advantages thereof, are also described in EP-B-0462165.
Typically the r.f. emitter means associated with the dielectric member is arranged to be connected to an r.f. power source which operates at a frequency in the range of from about 1 MHz up to about 45 MHz, e.g. at about 2 MHz or, more preferably, at one of the industrially allotted wavebands within this range of frequencies e.g. at 13.56 MHz or 27.12 MHz or 40.68 MHz.
By appropriate choice of geometry for the spiral driving coil and by modifying the magnetic field strength and/or distribution within the plasma chamber it is possible to tune the excitation of the discharge for a variety of gases, e.g. Ar, O2 or N2.
In a preferred form an ion gun according to the invention further includes secondary magnet means associated with the r.f. emitter means for producing a magnetic dipole field that penetrates the r.f. energising coil or other form of r.f. emitter means.
It is also possible to provide a further magnet means, hereinafter called a tertiary magnet means, for superimposing a longer range axial field on top of the field produced by the multipole array of said primary magnet means. Such a tertiary magnet means can, for example, take the form of an electromagnet surrounding the plasma chamber whose axis is arranged to be substantially aligned with or parallel to that of the plasma chamber.
In an ion beam apparatus according to the invention it is preferred to utilise an ion beam neutraliser that is powered by an r.f. energy source to produce a beam of electrons that can be projected into the ion beam. Conveniently, such an r.f. energy source operates at the same frequency as that of the r.f. generator means of the ion gun.
The invention thus may utilise an ion beam neutraliser comprising an open ended plasma source chamber, means for admitting a plasma forming gas to the plasma source chamber, an r.f. generating coil surrounding the plasma source chamber for generating a plasma therein, and an extraction grid structure across the open end of the plasma source chamber including a first grid arranged for connection to a negative potential source and a second grid arranged for connection to a positive potential source so as to produce a first acceleration field for accelerating electrons towards and through the second grid of the extraction grid structure. Such an ion beam neutraliser may use an inert gas, a reactive gas or a mixture of an inert gas and a reactive gas, as plasma forming gas.